A police officers rides on the back of a van at a staging area as a manhunt is conducted for a suspect Friday in Watertown, Mass. / Matt Rourke, AP

by Associated Press, USA TODAY

by Associated Press, USA TODAY

A massive police operation overnight has a Boston suburb on high alert.

A man police have described as a "terrorist" is being sought, while a second suspect is dead. They are the men believed to be involved in Monday's bombing at the Boston Marathon, authorties said.

The second suspect was believed to be on the loose in Watertown, Mass.

"It definitely sounded like it was a shootout/chase," said Watertown resident Michael McGee. "...A few explosions. It sounded kind of like pops."

Rusty Shakleford saw police activity and wanted to see what was happening. "I could tell they were serious because he drew a pistol at me and was like, go back, go home," Shakleford said of an officer.

Imran Saif, a Boston cab driver, said "It sounded like fireworks." "I heard a loud boom." He said he heard what he believed to be automatic weapons, and an explosion.

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